Friend helper virus induces a variety of hematopoietic neoplasms including erythroblastosis, myeloid leukemia, and T and B cell lymphomas. These neoplasms are clonal and contain Friend helper virus integrated into cellular DNA. Analysis of mouse strain differences in the type of leukemia induced by Friend helper virus has led to identification of a new Friend virus resistance gene designated Fhe(r) which makes C57BL mice resistant to Friend helper erythroblastosis. This gene acts at a step subsequent to Friend helper replication. Friend helper virus does not induce recombinant mink cell focus-forming viruses in C57BL/6 mice as it does in mice susceptible to Friend helper erythroblastosis. This finding opens up the possibility that cloning Friend helper virus-cell DNA junction fragments in C57BL myeloid leukemias and lymphomas will lead to the identification of unique cellular sequences which play a role in these tumors. AKR mice are also resistant to Friend helper virus erythroblastosis but develop accelerated thymic lymphomas after Friend helper virus inoculation. Genetic studies to identify AKR genes involved in resistance to Friend helper virus erythroblastosis are in progress.